Just hit this with the memcache import for keystone:

http://logs.openstack.org/76/644576/1/gate/grenade-py3/e8d5a3b/logs/screen-keystone.txt.gz?level=TRACE#_Mar_22_01_10_29_138043

Mar 22 01:10:29.138043 ubuntu-bionic-rax-dfw-0004128909 <email address hidden>[1887]: CRITICAL keystone [-] Unhandled error: ModuleNotFoundError: No module named 'memcache'
Mar 22 01:10:29.138043 ubuntu-bionic-rax-dfw-0004128909 <email address hidden>[1887]: ERROR keystone Traceback (most recent call last):
Mar 22 01:10:29.138043 ubuntu-bionic-rax-dfw-0004128909 <email address hidden>[1887]: ERROR keystone   File "/usr/local/bin/keystone-wsgi-public", line 54, in <module>
Mar 22 01:10:29.138043 ubuntu-bionic-rax-dfw-0004128909 <email address hidden>[1887]: ERROR keystone     application = initialize_public_application()
Mar 22 01:10:29.138043 ubuntu-bionic-rax-dfw-0004128909 <email address hidden>[1887]: ERROR keystone   File "/opt/stack/old/keystone/keystone/server/wsgi.py", line 24, in initialize_public_application
Mar 22 01:10:29.138043 ubuntu-bionic-rax-dfw-0004128909 <email address hidden>[1887]: ERROR keystone     name='public', config_files=flask_core._get_config_files())
Mar 22 01:10:29.138043 ubuntu-bionic-rax-dfw-0004128909 <email address hidden>[1887]: ERROR keystone   File "/opt/stack/old/keystone/keystone/server/flask/core.py", line 164, in initialize_application
Mar 22 01:10:29.138043 ubuntu-bionic-rax-dfw-0004128909 <email address hidden>[1887]: ERROR keystone     startup_application_fn=loadapp)
Mar 22 01:10:29.138043 ubuntu-bionic-rax-dfw-0004128909 <email address hidden>[1887]: ERROR keystone   File "/opt/stack/old/keystone/keystone/server/__init__.py", line 46, in setup_backends
Mar 22 01:10:29.138043 ubuntu-bionic-rax-dfw-0004128909 <email address hidden>[1887]: ERROR keystone     drivers = backends.load_backends()
Mar 22 01:10:29.138043 ubuntu-bionic-rax-dfw-0004128909 <email address hidden>[1887]: ERROR keystone   File "/opt/stack/old/keystone/keystone/server/backends.py", line 41, in load_backends
Mar 22 01:10:29.138043 ubuntu-bionic-rax-dfw-0004128909 <email address hidden>[1887]: ERROR keystone     cache.configure_cache()
Mar 22 01:10:29.138043 ubuntu-bionic-rax-dfw-0004128909 <email address hidden>[1887]: ERROR keystone   File "/opt/stack/old/keystone/keystone/common/cache/core.py", line 124, in configure_cache
Mar 22 01:10:29.138043 ubuntu-bionic-rax-dfw-0004128909 <email address hidden>[1887]: ERROR keystone     cache.configure_cache_region(CONF, region)
Mar 22 01:10:29.138043 ubuntu-bionic-rax-dfw-0004128909 <email address hidden>[1887]: ERROR keystone   File "/usr/local/lib/python3.6/dist-packages/oslo_cache/core.py", line 235, in configure_cache_region
Mar 22 01:10:29.138043 ubuntu-bionic-rax-dfw-0004128909 <email address hidden>[1887]: ERROR keystone     '%s.' % conf.cache.config_prefix)
Mar 22 01:10:29.138043 ubuntu-bionic-rax-dfw-0004128909 <email address hidden>[1887]: ERROR keystone   File "/usr/local/lib/python3.6/dist-packages/dogpile/cache/region.py", line 591, in configure_from_config
Mar 22 01:10:29.138043 ubuntu-bionic-rax-dfw-0004128909 <email address hidden>[1887]: ERROR keystone     "%sreplace_existing_backend" % prefix, False),
Mar 22 01:10:29.138043 ubuntu-bionic-rax-dfw-0004128909 <email address hidden>[1887]: ERROR keystone   File "/usr/local/lib/python3.6/dist-packages/dogpile/cache/region.py", line 423, in configure
Mar 22 01:10:29.138043 ubuntu-bionic-rax-dfw-0004128909 <email address hidden>[1887]: ERROR keystone     _config_prefix
Mar 22 01:10:29.138043 ubuntu-bionic-rax-dfw-0004128909 <email address hidden>[1887]: ERROR keystone   File "/usr/local/lib/python3.6/dist-packages/dogpile/cache/api.py", line 89, in from_config_dict
Mar 22 01:10:29.141398 ubuntu-bionic-rax-dfw-0004128909 <email address hidden>[1887]: ERROR keystone     for key in config_dict
Mar 22 01:10:29.141398 ubuntu-bionic-rax-dfw-0004128909 <email address hidden>[1887]: ERROR keystone   File "/usr/local/lib/python3.6/dist-packages/dogpile/cache/backends/memcached.py", line 208, in __init__
Mar 22 01:10:29.141398 ubuntu-bionic-rax-dfw-0004128909 <email address hidden>[1887]: ERROR keystone     super(MemcacheArgs, self).__init__(arguments)
Mar 22 01:10:29.141398 ubuntu-bionic-rax-dfw-0004128909 <email address hidden>[1887]: ERROR keystone   File "/usr/local/lib/python3.6/dist-packages/dogpile/cache/backends/memcached.py", line 108, in __init__
Mar 22 01:10:29.141398 ubuntu-bionic-rax-dfw-0004128909 <email address hidden>[1887]: ERROR keystone     self._imports()
Mar 22 01:10:29.141398 ubuntu-bionic-rax-dfw-0004128909 <email address hidden>[1887]: ERROR keystone   File "/usr/local/lib/python3.6/dist-packages/dogpile/cache/backends/memcached.py", line 287, in _imports
Mar 22 01:10:29.141398 ubuntu-bionic-rax-dfw-0004128909 <email address hidden>[1887]: ERROR keystone     import memcache  # noqa
Mar 22 01:10:29.141398 ubuntu-bionic-rax-dfw-0004128909 <email address hidden>[1887]: ERROR keystone ModuleNotFoundError: No module named 'memcache'
Mar 22 01:10:29.141398 ubuntu-bionic-rax-dfw-0004128909 <email address hidden>[1887]: ERROR keystone